Interference is one of major causes of performance degradation in a wireless network environment. Particularly, such an interference problem is mainly caused in a multi-user environment in which several transmitting-receiving pairs exist. This is because a transmission signal simultaneously transmitted from each transmitter is delivered to non-intended receivers as well as to a target receiver. Many attentions have been paid to an interference alignment (IA) technology as a potential solution method for solving such an interference problem. The interference alignment technology was originally developed under the model of an interference channel between X-channel and K-user, and has been developed for various practically-relevant network models. Particularly, an interference alignment technology, which is capable of achieving the almost same performance as DoF (degree of freedom) performance under interference-free circumstances, under cellar network circumstances has been developed.
Although it is true that such interference alignment technologies contribute to increasing communication capacity, there exist various tasks to apply them to a real environment. One of the tasks is that a transmitter should be aware of current channel state information (CSI). In a related art FDD (frequency division duplex) communication system, as such channel informations are obtained from feedback from a receiver to a transmitter, a delay is generated due to additional requirement for receiving feedback. Hence, in a related art communication system, after predicting channel information based on such outdated channel state information (outdated CSI), an interference alignment technology is applied based on the predicted channel information. Yet, in a fast-fading scenario in which a channel environment changes fast, a current channel state may be totally different from a channel state predicted from feedback. In this case, a method of predicting a current channel state fails in improving communication capacity (DoF gain).
Yet, it turns out that communication capacity on multi-antenna broadcast channel can be improved with outdated channel state information. Namely, in a fast-fading environment in which a channel environment changes fast, although channel information fed back from a receiver is totally different from a current channel state, it is known that efficiency of communication can be improved using such channel information. If communication performance under such extreme circumstances is improved to be better than that of a case of no channel information, efficiency of communication can be improved in any environments using delayed outdated channel information.